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Better to overcool than undercool. A CPU survives best with adequate cooling to


John Lamprou
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Hackability of the CPU and system board is not the only consideration for a CPU speed tweak. As the CPU goes faster, the internal temperature rises, stressing the incredibly small wires and component structures inside. With excessive heat comes random lockups of the system and possibly catastrophic failures, with some spectacular but short-lived fireworks as the CPU melts down. To counteract excessive heat requires significant cooling capability attached to the CPU chip, so you will see a lot of heat-sink and cooling fan gimmicks and gadgets for sale with CPUs. Check the documentation that comes with the CPU chips, and you will find recommendations and warnings about ensuring proper CPU-to-heat-sink contact and adequate ventilation. Figure 3-10 shows an example of a specially milled heavy-duty supercooling heat sink from an HP server with an integrated fan. HP engineers lay claim to inventing this style of cooling device, and it either works very well or just looks cool as heck! This design has been cloned by many aftermarket vendors.

 

    WARNING: Never run your CPU without a heat sink, especially the ultra-hot AMD processors!

 

Anyone who has run an AMD Athlon or Duron CPU—any version at any speed, overclocked or not—will tell you that the chip must be fitted with a decent heat sink and fan before any power is turned on, or the CPU will almost certainly fail. Figure 3-11 shows two CPU chips that have suffered catastrophic thermal failure when operated without a heat sink. Try as you might, you cannot put the “magic smoke” back in the chip and have it work again.

 

    WARNING: Avoid inhaling the smoke or fumes from a “flamed-out” chip. When the internal elements of a CPU or other semiconductor melt or burn, they give off very foul-smelling and possibly toxic fumes. If a CPU does burn up, ventilate the area well to clear the air and be wary of nausea, dizziness, or other ill effects of toxic contamination.The stock heat sink that comes with your CPU is adequate for operating the CPU at its rated speed, but overclocking and voltage adjustments can raise CPU temperature dramatically. In most cases of moderate (10–20%) clock or voltage (5–10%) increase, a slightly bigger heat sink and better ventilation will suffice to keep the chip temperature within safe operating range. In the rare cases when you can kick the CPU speed up by 25–200% or more, you need to provide some serious heat removal.

 

Current CPU types provide internal temperature sensors that can be read by the system BIOS and by some utility programs like SiSoft Sandra. Reading the temperature of your system running normally will give you a baseline operating temperature to compare with as you overclock. You must avoid reaching or exceeding the thermal limits of your CPU.

 

Although the maximum idle temperature on many AMD CPUs can be as high as 95 degrees Centigrade, the actual running temperature is 30–40 degrees C. Many BIOS versions provide CPU temperature alarms at 60, 65, and 70 degrees C. Your heat sink and ventilation should keep the CPU’s running temperature well below 60 degrees C, and you should overclock your CPU no more than 25%. The secret to heat removal is to have a large mass of material with low thermal resistance to conduct heat away from the chip into the surrounding cooler air. Alternatively, you can attach a device with a circulating coolant that draws the excess heat away quickly and dumps away from the system components, like the radiator in your car or home air conditioner does.

 

Aluminum is the ideal metal for most heat sinks. It has low thermal resistance, so it can accept and dissipate thermal energy very efficiently. It is inexpensive and easily manufactured into a variety of shapes that provide fast thermal dissipation and contact with almost any surface that needs cooling. Copper, also used in some heat sinks, is more expensive but is the material of choice for water-cooled devices.

 

In addition to using a highly thermal-conductive material, that material must be as tightly attached to the CPU as possible. It is not adequate to merely place the material next to the CPU: the bond must be as close to being a part of the CPU as possible. The bond is usually made with a very thin layer of thermally conductive grease or epoxy adhesive specifically for heat-sink bonding.

 

The layer must be very thin because the compound or adhesive is intended to improve the metal-to-metal contact by filling in minute imperfections in both surfaces to provide optimal contact and thermal transfer. If the layer can be made thin enough, it will only cover the imperfections and leave metal-to-metal contact at the high spots common to both surfaces.

 

    TIP: An often neglected attribute of using a thin layer of thermal compound is that it eliminates air bubbles between the surfaces that may trap a small amount of moisture. Moisture trapped between a hot device and its heat sink does not constitute water cooling; instead it could be a small water bomb waiting to go off. See Step 11 in the instructions that follow.

 

    If the temperature of the moisture bubble exceeds 100 degrees Celsius or 212 degrees Fahrenheit (the boiling point of water)—and it can with a souped-up CPU—the water will expand to 2,700 times its volume as steam and demand to go someplace. That will likely be in the direction of destroying the CPU chip or at least weakening the overall thermal bond, causing the CPU to overheat and self-destruct.

 

    If you’ve removed a heat sink from a CPU (best done with either a slight twisting motion to separate heat sink and CPU or very light prying between the two), beware that some are glued on with high-temp epoxy and cannot be removed without destroying the CPU. You’ve probably experienced this thermal grease or heat-sink compound—a tenacious white material that looks and feels like toothpaste but stains like red wine in the middle of a new white carpet. Thermal compound is typically a mixture of aluminum oxide for thermal conduction and a silicon paste to hold the aluminum oxide tpgether

Two new compound mixtures have emerged: one containing aluminum oxide in a fine ceramic form, the other silver and silver oxide. According to product documentation at http://www.articsilver.com, the typical aluminum oxide–based white paste provides the lowest thermal conductivity and the least CPU temperature drop (2–7 degrees), the ceramic compound is next in the order of effectiveness (2–10 degree drop), and the silver-based compound the most efficient, providing a 3–12 degree drop in CPU temperature. The effectiveness is also represented in the cost of the compound— between $4 and $9 per tube. Unless you see your CPU temperature rising towards its maximum limits, the typical aluminum oxide, and certainly the ceramic paste, are more than adequate for the task.

 

    TIP: For a time, Intel prebonded heat sinks to some versions of their Pentium I CPUs using thermal epoxy, making it impossible to separate the two if you wanted to add a larger heat sink.

 

To speed production processes and make applying thermal bonding cleaner, many vendors have chosen to use thermal pads, as shown in Figure 3-13. Thermal pads are fine in lower-temperature applications, but, while they certainly fill gaps between surfaces, they do not give way to allow direct surface contact between high spots. If you separate a CPU and heat sink that were bonded with a thermal pad, it is acceptable to replace the pad with thermal paste instead, unless the warranty on your CPU requires the use of the supplied thermal pad and heat sink.

o matter which compound you choose, the technique for properly applying thermal compound to obtain optimal thermal bonding between a cooling device and a CPU involves a few very simple items and steps.

 

    * What you will need (see Figure 3-14): Thermal compound

    * A clean, dry cloth, something as lint-free as possible

    * Isopropyl (rubbing) alcohol

    * A vinyl glove or piece of plastic wrap

    * A straightedge, such as a single-edged razor blade or used plastic card

    * An antistatic pad or chip storage bag to pad the CPU pins and reduce the chance of static damage

Use these items to install your heat sink as follows:

 

  1.

 

      Remove the CPU from its socket and set it pins-down on the antistatic material.

  2.

 

      Maintain cleanliness! Apply a few drops of isopropyl alcohol to the clean cloth and wipe the contact surface area of your heat sink and the top cap of the CPU core. Alcohol will remove most oils and help evaporate moisture from the surfaces.

  3.

 

      Apply a small bead/drop of thermal compound to the area of the heat sink that will contact the CPU, as shown in Figure 3-15.

  4.

 

      Protecting your fingers with the vinyl glove or plastic wrap, smear the compound around and into the surface of the heat sink, as shown in Figure 3-16. This will help fill imperfections in the metal surface.

  5.

 

      Using a clean, dry portion of the cloth, wipe the excess thermal compound off the surface of the heat sink, as shown in Figure 3-17. If the compound is especially thick and hard to wipe off, scrape the excess off with the straightedge and then wipe clean. You should not expect to remove all evidence of the compound, but leave minute amounts on the surface. Do not use alcohol to clean the surface.

#

 

Apply a small bead of thermal compound to a corner of the CPU’s metal die/cap, as you did in Step 1.

#

 

Using the straightedge, distribute the compound evenly across the surface of the top of the CPU

Remove as much excess as possible but leave a thin layer of compound

#

 

Install the CPU in its socket on your system board. Be careful not to disturb the thermal compound.

#

 

Align and place the heat sink as squarely and accurately in its final placement above the CPU as possible.

#

 

Apply a slight downward pressure evenly on the heat sink, then twist the heat sink to the left and right of its final placement position and back to its final centered position, as in Figure 3-20. This action will press out excess compound and fill in any gaps, reducing any bubbles and the surface-to-surface distance between the heat sink and CPU.

#

 

Secure the heat sink in place with its bracket (usually clipping in the back-end bracket slots and then the side with the “handle”), plug in the fan if your heat sink is equipped with one, and begin to enjoy your cooler CPU.

ollow the directions carefully for heat-sink fastening. The mechanics and fastening system for your heat sink, CPU, and system-board socket may be different than the one shown.

 

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